1. Field of the Disclosure
The disclosure is related to the field of electromagnetic induction well logging for determining the resistivity of earth formations penetrated by a wellbore. More specifically, the disclosure relates to measuring the transient signals in an induction tool having a metallic pipe with finite, non-zero and high conductivity.
2. Description of the Related Art
Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al. The instrument described in the Beard '761 patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages which are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
The development of deep-looking electromagnetic tools has a long history. Such tools are used to achieve a variety of different objectives. Deep looking tools attempt to measure the reservoir properties between wells at distances ranging from tens to hundreds of meters (ultra-deep scale). There are single-well and cross-well approaches, most of which are rooted in the technologies of radar/seismic wave propagation physics. This group of tools is naturally limited by, among other things, their applicability to only high resistivity formations and the power available downhole.
Deep transient logging while drilling (LWD), especially “look-ahead” capability, was shown to have a great potential in predicting over-pressured zones, detecting faults in front of the drill bit in horizontal wells, profiling massive salt structures, etc. One of the main problems of deep transient measurements in LWD application is a parasitic signal due to the conductive drill pipe. A variety of techniques have been used to reduce this parasitic signal in the acquired data. For the purposes of the present disclosure, we adopt the following definition of the term “Transient Electromagnetic Method” from the Schlumberger Oilfield Glossary:                A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current.        
Among the methods that have been used to reduce the parasitic signal due to a conductive drill pipe are using ferrite and copper shielding, using a reference signal (bucking) for calibration purposes, and using the asymptotic behavior of the conductive pipe time response to filter out the pipe signal.
U.S. Pat. No. 7,027,922 to Bespalov, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference is of particular interest. As disclosed in Bespalov, the transient signal may be represented by the Taylor Series expansion:
            (                                                                  H                z                            ⁡                              (                                  t                  1                                )                                                                                                        H                z                            ⁡                              (                                  t                  2                                )                                                                          ⋮                                                                              H                z                            ⁡                              (                                  t                                      m                    -                    1                                                  )                                                                                                        H                z                            ⁡                              (                                  t                  m                                )                                                        )        =.    ⁢      (                                        t            1                                          -                1                            /              2                                                            t            1                                          -                3                            /              2                                                            t            1                                          -                5                            /              2                                                …                                      t            1                          n              /              2                                                                        t            2                                          -                1                            /              2                                                            t            2                                          -                3                            /              2                                                            t            1                                          -                5                            /              2                                                …                                      t            2                          n              /              2                                                            ⋮                          ⋮                          ⋮                          …                          ⋮                                                  t                          m              -              1                                                      -                1                            /              2                                                            t                          m              -              1                                                      -                3                            /              2                                                            t                          m              -              1                                                      -                5                            /              2                                                …                                      t                          m              -              1                                      n              /              2                                                                        t            m                                          -                1                            /              2                                                            t            m                                          -                3                            /              2                                                            t            m                                          -                3                            /              2                                                …                                      t            m                          n              /              2                                            )    ⁢      (                                        S                          1              /              2                                                                        S                          3              /              2                                                                        S                          5              /              2                                                            ⋮                                                  S                                          (                                                      2                    ⁢                                                                                  ⁢                    n                                    -                  1                                )                            /              2                                            )  Where Hz is the z-component of the magnetic field, t is the time and the S-s are expansion coefficients. As discussed in Bespalov, the S1/2 and S3/2 terms are dominated by the effects of the conductive pipe, and estimating and correcting for at least the S1/2 component and, optionally, also the S3/2 component gives a transient response that is sensitive to the distance to bed boundaries.
In case the target DOI of up to 50 meters the conductive pipe signal is typically more than two orders of magnitude greater than the formation signal even if the ferrite and copper shields are used. Under these conditions, the accuracy of bucking (e.g. due to exposure to the down-hole conditions), and asymptotic filtering may not be sufficient to facilitate measurements. The present disclosure addresses the problems for extra deep resistivity measurements.